The invention relates to a silo for pulverulent and finegrained bulk and other loose materials having a conical cover in the center of a circular silo base, the base being provided with pneumatic fluidizing means and being slightly inclined towards material outlets formed therein.
Such silos or hoppers, e.g., are used for the storage of raw flour, cement, fly ash, coal dust and gypsum. Known material emptying or discharging means for silos of this type are described in German publications DE-AS 23 52 455 and DE-AS 25 47 667. Such silos are centrally provided with a conical silo base along which the sinking loose material slides into the outer ring base zone. The circular silo base between the central cone and the silo outer wall is inclined towards the particular outlet openings and is provided with material loosening mechanisms. An outflow towards the outlet openings takes place through venting the loose material in the base zone.
Unfortunately, the venting only takes place in a very incomplete manner. As a result of the high compression pressure of the complete loose material column in the silo, the material is compressed to such an extent that the loosening air on the silo base only acts in the immediate vicinity of the outlet openings. As the air cannot escape upwards through the loose material in the silo, the entire loosening air quantity must escape through the outlet openings together with the loose material.
During the emptying or discharging process, the loose material column immediately above the outlet opening is subject to movement. This column assumes a gradually enlarging funnel shape upwards towards the loose material level. Therefore, there are dead silo zones not participating in the movement and this material is very strongly compressed and formed into lumps over a long period, sometimes several years. If it is necessary to almost completely empty a silo, i.e. the loose material level is moved further downwards, then from the previously passive silo zones there gradually is a detachment of the loose material layers, which then approach the outlet zone generally in the form of lumps. This lumping action contributes to the clogging of the outlet zone.
The very complicated arrangement of a plurality of outlet connections with connected material dosing means is also unable to prevent the phenomenon of passive silo zones.
Furthermore, German utility model publication 75 23 514 discloses a container base for silos of the present type, which has a plurality of pneumatic feed channels passing from the circumference to the material outlet and which subdivide the container into sector-like portions. Over each pneumatic feed channel is provided a ventilatable cover inclined towards the outer circumference of the container base. This measure is intended to prevent material sinking difficulties and provide a uniform sinking of the material column during discharge from the silo. However, this can only take place in a very small area, namely in the immediate influence area of the feed channels or the ventilatable cover positioned above them.
A primary object of the present invention is to provide a silo of the aforementioned type, in which the loose materials stored in the silo uniformly drop downwards in the form of a "mass flow", i.e. uniformly over the silo cross-section, thus preventing dead or passive zones in the silo.
To achieve this object of the invention, a second or upper circular silo base is located at a distance above the lower circular silo base between the conical cover and the silo outer wall for forming a separate material collecting chamber. A plurality of radially positioned slots are formed in the upper silo base and fluidizing means are provided adjacent thereto. The annular space forming the collecting chamber is connected beneath the upper base with a venting line.
Advantageously, the cross-sections of the slots of the upper silo base are regulatable, i.e. adjustable from zero to a maximum desired cross-section, so as to directly influence the "mass-flow" of the material from the upper silo storage area into the material collecting or outlet chamber.
For all the surface areas of the silo base and because of the plurality of radially arranged slots, there is the same pressure loss for the fluidizing layer activated on the base. The vertically directed mass flow can uniformly act and be collected in the underlying outlet chamber. Only limited air pulses with a fraction of the energy are required compared with the conventional silo pneumatic material emptying means. The slots in the upper base take over the function of indirect silo outlets and transfer the loose material over the shortest route with the minimum pressure loss into the material outlet chamber.
Surprisingly, the material outlet chamber is not overfilled even during the loosening pulses on the upper silo base above it. Moreover, by covering the slots and/or using fittings inhibiting the vertical loose material flow, resistances are provided which intensely support the ventilating of the loose material in the frictional resistance process, particularly after the end of the brief pulsed ventilation and consequently prevent an afterflow into the outlet chamber. The chamber volume is dimensioned in such a way that there is an adequate capacity. Preferably the chamber volume is 2.5 to 5% of the total volume of the silo.
The ventilation of the chamber ensures a pressure compensation to the free silo upper area. As the relief paths of the loosened bulk material on the upper ring silo base are the same for all surface areas and are extremely short due to the slot arrangement with limited spacing intervals, it is sufficient, e.g., for cement loosening to release the frictional engagement of the stored material with compressed air having a pressure difference of only 200 to 300 mbar.
Conventional silo material emptying means, in which the loose material must cover longer horizontal flow paths to the constricted silo material outlet, make it necessary to design the compressed air means with a pressure differences of 400 to 800 mbar.
Recently, there have even been material emptying systems which operate with pressure differences up to 7.5 bar for loosening in storage silos, cf. the journal Zement Kalk Gips, No. 11/86, where on pp 596/7 a silo with a diameter of 18 meters is described for cement.
The considerable higher energy requirements result from the previously described strong and in particular unequal loose material densities in different silo zones. It is known in connection with loose materials that in the case of long storage periods, i.e. several months or years, unequal loose material densities on the silo base cannot be broken up even with increased pressure energy. These prior art disadvantages are eliminated by the invention because there is achieved a surface-equal material mass flow.
A further advantage of the invention is the improvement to the quality of the stored loose material through the increased mass exchange within the mass flow funnels produced. Whereas in the case of through-flow mixing silos according to German publication DE-AS 23 52 455/25 47 667, the mixing funnels produced comprise a long, narrow funnel neck and a funnel "tulip: located in the material level, the mass flow funnels are comparable with an inverted frustum shape. The action of the silo base surface extends over the actual surface area of the material loosening section. Through the changing venting of the material loosening sections and the described spatial extension of the mass flow funnels during material removal, intermeshing flow profiles act over the entire silo content. Unlike in the known silo types, strong composition fluctuations cannot break through to the lower silo base outlet and are instead dampened. The upper silo base therefore constitutes the actual operational base for the described advantageous process sequence. Material loosening appropriately takes place through use air troughs adjacent the radially arranged slots of the upper silo base.
Domes or slopes which are arranged above the slots of the upper silo base can prevent a short-circuit feed into the outlet chamber, because the unventilated material, compressed by support on the silo wall and base, is unable to convert inoperative frictional resistance forces into horizontal forces. Moreover, the outlet chamber is permanently vented, unlike the silo sections above the upper base.
It is also advantageous to provide additional fittings for the upper silo base inhibiting the loose material flow. These fittings are directly incorporated into the slots of the upper base. It is possible to use air feed troughs, loosening pipes and pivotable baffle plates. As a function of the operating state, the through-flow resistances can be increased or decreased. The cross-sections of the slots are so constructed that blockages are avoided. In the case of concrete bases, the slots are widened downwards. However, this can be avoided with steel structures.
In the case of a corresponding construction having baffle plates, they can also be used for closing the upper base. Therefore, if necessary the outlet chamber can be inspected even when the upper silo storage area is filled.
It is also conceivable for certain loose materials and silo sizes to cover the slots directly by a corresponding arrangement of the air feed troughs provided on the upper base.
For loose materials with good flow characteristics and advantageous storage conditions, the loosening surface of the upper base can also be halved, in that slopes in the form of concrete slides alternate with the loosening sections. In an advantageous construction, they cover the slots.
Generally, it is only necessary to energize one loosening section on the upper base by means of a short air pulse. The pulse time cycle is a function of the level of the quantity flow taken from the silo, as well as the flow behavior of the loose material. The filling state of the outlet chamber is indicated by the counterpressure in the ventilation system and can consequently be used in a simple manner as a control.
The construction of the silo base in accordance with the invention makes it possible to reduce to a minimum the costs for concrete. Merely through drawing in the upper base, an outlet chamber is formed with no significant additional costs compared with the complex system and other arrangements of the comparable prior art.
The simple operation of the silo mass flow material emptying means is characterized by only two silo outlet connections with corresponding material blocking and dosing means, independently of the size of the silo diameter.
The connection feed can take place directly from these silo outlets, because the outlet chamber simultaneously fulfills a material collecting and distributing function. In the case of a silo construction according to German publication DE-AS 25 47 667, behind the silo outlet connection is arranged an additional material container with further material dosing means for distribution over connecting feeds.